A semiconductor device in which a cooler is pressed against a surface of a semiconductor module containing a semiconductor element is known. Some examples of this type of semiconductor device are disclosed in Japanese Patent Application Publication No. 2007-173372 and Japanese Patent Application Publication No. 2001-267475. In semiconductor modules disclosed in the aforementioned documents, a heat sink is exposed on a part of one surface of a resin package containing a semiconductor element. The exposed surface of the heat sink serves as a thermal conductive surface. The cooler is pressed against the semiconductor module while the cooler faces the thermal conductive surface of the heat sink and a resin surface around the thermal conductive surface. In the semiconductor device disclosed in Japanese Patent Application Publication No. 2007-173372, grease (non-solid heat transfer material) is filled between the semiconductor module and the cooler. The grease enters into between fine irregularities on a surface of the cooler and also enters into between fine irregularities on the thermal conductive surface of the heat sink, which results in a rise in thermal conductivity from the heat sink to the cooler. The grease layer is spread thin by pressing the cooler on the semiconductor module.
A temperature of the heat sink repeatedly rises and falls according to a heat cycle of the semiconductor element. When the heat sink thermally expands, the thermal conductive surface of the heat sink deforms such that a center of the thermal conductive surface of the heat sink is swollen. Accordingly, the grease is pushed out onto the resin surface around the thermal conductive surface of the heat sink. Once the heat sink cools down, the thermal conductive surface is recovered to a flat state. However, part of the grease may not return to its original position. That is, the grease may sometimes flow out from a portion between the thermal conductive surface of the heat sink and the cooler to its surrounding portion over the course of the semiconductor element repeating the heat cycle. Notably, for explanation convenience, a phenomenon where grease flows out from between the thermal conductive surface of a heat sink and a cooler to its surrounding portion will be hereinafter referred to as “grease dissipation”.
Meanwhile, the semiconductor device of Japanese Patent Application Publication No. 2001-267475 includes a solid heat transfer layer between a thermal conductive surface of a heat sink and a cooler. Since the solid heat transfer layer does not have fluidity, the aforementioned grease dissipation may not occur. Notably, in the semiconductor device of Japanese Patent Application Publication No. 2001-267475, a part of the heat sink protrudes from a resin surface around the heat sink, and the heat transfer layer is interposed between the cooler and a top surface of the protruding portion of the heat sink. Since the part of the heat sink protrudes from the resin surface, the resin surface is set apart from the cooler and the heat transfer layer is not provided between the resin surface and the cooler.